1. Field of the Invention
The invention relates to a device for fusing and fixing a toner image on a carrier in an electromagnetic radiation field, with a uniformly radiating microwave power transmitter connected to a microwave generator and extending across the web width of the carrier.
2. The Prior Art
Such a fixing device working with microwaves is known from German Offenlegungsschrift No. 1,497,204 and consists of a rod-shaped slot radiator which, for the production of a narrow beam of radiation, is embedded in a partly cylindrical reflector, made up of dielectric material, which narrows in the direction towards the image carrier to a belt-shaped radiation aperture. Opposite the radiation exit aperture of this reflector an additional reflector is provided, on the other side of the paper web, to screen or reflect the radiation.
With this device the slot radiator has to be run with very high power in order to deliver sufficient energy for the fusing of the toner particles since in each case only a strip of the paper web, corresponding to the width of the radiation aperture of the reflector, is irradiated for a very short period. A sufficiently uniform fixing is in this case not always guaranteed since the frequency variations of the single microwave radiator manifest themselves completely as variations in the radiated energy.
Further microwave fixing devices are described in U.S. Pat. No. 3,462,285 and consist either of a closed inductance heating loop connected to a high frequency source, through which loop the film with the toner image is fed, or of a dielectric heating device which has an upper and a lower plate on the two sides of the film with the toner image.
These known devices are unsuitable for very wide sizes since, because of the single radiator, the sinusoidal spread of amplitude across the width of the film carrying the image or of the paper web falls away strongly towards the edges and thus the fixing becomes weaker towards the edges.
To achieve a homogeneous spread of amplitude in microwave continuous equipment for the heating of non-metallic material, a transverse radiator is known from German Offenlegungsschrift No. 1,565,266 in which several short-circuited E-sector transverse (magnetic waves) horns in a row next to one another extend across the width of the material to be heated.
The paths or walls betwen the E-horns produce unheated, or only slightly heated, areas on the material passing below.
In the field of electrophotography it is necessary to fix the toner image produced on a carrier. From the point of view of energy requirement, because of the favorable heat transfer, contact fixing represents a favorable process which, however, has the disadvantage that the toner particles caused to fuse remain stuck on the contact-making roll in the form of the image and are transferred onto the next copy in the form of so-called "ghost images", unless appropriate separation and cleaning means are provided to clean the contact-making roll after the transfer. These means increase the construction expenses and do not contribute to an increase in operational reliability.
In addition to contact fixing, contactless fixing means are also used such as, for example, single radiators with focusing reflectors, radiator groups with simple metal sheet reflectors, and flashlight radiators.
These fixing devices have to be fed with more power than is necessary for the fusing of the toner particles on the carrier material. Because of the high element temperatures of such radiators, for example infra-red radiators, which can be between 300.degree. and 400.degree. C., there exists considerable danger of fire for combustible carrier materials such as paper. In particular, in the case of a carrier material made of paper becoming stuck during fixing in the fixing device the danger of fire is very great. The danger of fire in that case cannot even be eliminated merely by switching off the radiator since the coiled filaments of the infra-red radiator, for example, afterglow and can still transfer sufficient energy to the carrier material to set the latter on fire.
Attempts to prevent high heat loading on copying machines which results from the fixing unit by the use of ventilation fans or suction fans are expensive and not very successful since the heat energy removed heats up the room in which the copying machine is placed in an undesirable manner and hence the heat loading on the copying machine does not become less.
Both contact fixing and also contactless fixing with the aid of radiators have the disadvantage of a slow volume heating, which is equivalent to a certain pre-heating time, the impossibility of an immediate start, and a sluggish reaction of the copying machine. Fixing with microwaves avoids these disadvantages in known fixing means since the energy transfer by the dielectric heating of non-metallic materials is effected by the direct conversion of the electromagnetic energy of the microwaves in the toner and paper. This energy transfer is based on the interaction of polar molecules or polar molecule groups with the alternating electric field of the microwaves. If a paper standstill occurs during the fixing operation within the fixing unit, the resulting switching off of the power supply to the microwave radiator also causes an immediate interruption of the energy transfer. There is no action which like the after-cooling of the radiator gives rise to a further energy transfer even after the standstill of the copying machine. A fire danger is thus to a large extent excluded.